COX-2 inhibitor

COX-2 selective inhibitor is a form of non-steroidal anti-inflammatory drug (NSAID) that directly targets COX-2, an enzyme responsible for inflammation and pain. Targeting selectivity for COX-2 reduces the risk of peptic ulceration, and is the main feature of celecoxib, rofecoxib and other members of this drug class. After several COX-2 inhibiting drugs were approved for marketing, data from clinical trials revealed that COX-2 inhibitors caused a significant increase in heart attacks and strokes, with some drugs in the class having worse risks than others. Rofecoxib (commonly known as Vioxx) was taken off the market in 2004 because of these concerns and celecoxib and traditional NSAIDs received boxed warnings on their labels.

Analysis of clinical trial data revealed that there was a significant increase in the rate of vascular events with COX-2 inhibitors compared with placebo; "vascular events" are non-fatal myocardial infarction (MI) or heart attack, non-fatal stroke, and death from a vascular event such as MI or stroke.[5][6] These results led Merck to voluntarily withdraw (Rofecoxib) from the market in September 2004 and to regulatory authorities imposing a boxed warning on the label of celecoxib.[5] Traditional NSAIDs were also found to have cardiovascular risks, leading to similar boxed warnings.[5]

The cause of the cardiovascular problems became, and remains, a subject of intensive research.[7] As of 2012 results have been converging on the hypothesis that the adverse cardiovascular effects are most likely due to inhibition of COX-2 in blood vessels, which leads to a decrease in the production of prostacyclin in them. Prostacyclin usually prevents platelet aggregation and vasoconstriction, so its inhibition can lead to excess clot formation and higher blood pressure.[7]

A model comparing the theoretical relative frequency of gastrointestinal adverse effects and cost effectiveness of celecoxib, nonspecific NSAIDs alone, NSAIDs plus a proton pump inhibitor, NSAIDs plus an H2 receptor antagonist, NSAIDs plus misoprostol, and diclofenac/misoprostol, found the lowest probability of adverse gastrointestinal events for celecoxib, followed by NSAIDs plus a proton pump inhibitor, NSAIDs plus an H2 receptor antagonist, NSAID plus misoprostol, diclofenac/misoprostol, and NSAID alone. In total cost, including drug plus treatment of any gastrointestinal effects, the lowest cost treatment was celecoxib, followed by NSAIDs alone and diclofenac/misoprostol, with the other NSAID plus gastrointestinal protection regimens being much more costly. Similarly, a model of cost effectiveness of rofecoxib and celecoxib compared to high-dose acetaminophen or ibuprofen, with and without misoprostol, in patients with osteoarthritis of the knee found that acetaminophen had the lowest cost for average patients. For those not responding to paracetamol, ibuprofen was the most cost effective treatment by a large margin, but for those who did not respond to acetaminophen and had a high risk of gastrointestinal damage, rofecoxib was the most cost effective treatment.

The adverse effects of COX 2 inhibitors are the subject of extensive scientific debate. Several studies have pointed to a subtle increase in the risk of myocardial infarction, hypertension and also potential gastrointestinal complications. The following examples illustrate some of these risks, however (as with all medications) a consultation with a medical professional should be sought to discuss these issues before a firm opinion regarding their safety is established.

A French study of osteoarthritis patients over 65 years of age determined that, compared to Celebrex (200 mg once daily), patients taking Vioxx (25 mg once daily) suffered a two-fold increase in clinically significant edema and 60% more frequent increases in systolic blood pressure greater than 20 mmHg, as early as the second week of treatment. This has significant implications, since it has been estimated that every 2 mmHg increase in blood pressure raises the risk of stroke by two thirds and the risk of myocardial infarction by one third, suggesting that Celebrex may be a better choice for hypertensive patients or those at risk for edema. In addition, COX-2 inhibitors lack some of the platelet inhibiting properties of aspirin and other nonspecific NSAIDs and may, directly or indirectly, lead to increased risk of thrombosis, particularly in high risk patients where low dose aspirin therapy is warranted. On the other hand, this property makes them a better choice for perisurgicalpain management, where inhibition of blood clotting would be problematic.

There are other differences between Celebrex and Vioxx that influence prescribing practices. Patients with known sensitivity to sulfa drugs are likely to be sensitive to Celebrex as well, due to similarity in structure. Vioxx has a more rapid onset and is approved for acute pain as well as osteoarthritis, while Celebrex is approved for rheumatoid arthritis as well as osteoarthritis.

COX-2 appears to be related to cancers and abnormal growths in the intestinal tract. COX inhibitors have been shown to reduce the occurrence of cancers and pre-cancerous growths. The National Cancer Institute has done some studies on COX-2 and Cancer.[8] And the FDA has approved Celebrex for treatment of familial adenomatous polyposis (FAP) [2]. COX-2 inhibitors are currently being studied in breast cancer[9] and appear to be beneficial.[10]

COX-2 inhibitors have been found to be effective in suppressing inflammatory neurodegenerative pathways in mental illness, with beneficial results in trials for major depressive disorder as well as schizophrenia.[11]

The inhibition of COX-2 is paramount for the anti-inflammatory and analgesic function of the selective COX-2 inhibitor celecoxib. However, with regards to this drug’s promise for the therapy of advanced cancers, it is unclear whether the inhibition of COX-2 plays a dominant role, and this has become a controversial and intensely researched issue. In recent years, several additional intracellular components (besides COX-2) were discovered that appear to be important for mediating the anticancer effects of celecoxib in the absence of COX-2.[12] Moreover, a recent study with various malignant tumor cells showed that celecoxib could inhibit the growth of these cells, even though some of these cancer cells didn’t even contain COX-2.[13]

Additional support for the idea that other targets besides COX-2 are important for celecoxib’s anticancer effects has come from studies with chemically modified versions of celecoxib. Several dozen analogs of celecoxib were generated with small alterations in their chemical structures.[14] Some of these analogs retained COX-2 inhibitory activity, whereas many others didn’t. However, when the ability of all these compounds to kill tumor cells in cell culture was investigated, it turned out that the antitumor potency did not at all depend on whether or not the respective compound could inhibit COX-2, showing that inhibition of COX-2 was not required for the anticancer effects.[14][15] One of these compounds, 2,5-dimethyl-celecoxib, which entirely lacks the ability to inhibit COX-2, actually turned out to display stronger anticancer activity than celecoxib itself [16] and this anticancer effect could also be verified in highly drug-resistant tumor cells[17] and in various animal tumor models.[18][19]

The COX-2 enzyme was discovered in 1988 by Daniel Simmons, a Brigham Young University researcher.[20] The mouse COX-2 gene was cloned by UCLA scientist Dr. Harvey Herschman, a finding published in 1991.[21]

The basic research leading to the discovery of COX-2 inhibitors has been the subject of at least two lawsuits. Brigham Young University has sued Pfizer, alleging breach of contract from relations BYU had with the company at the time of Dr. Simmons work.[22][23] A settlement was reached in April 2012 in which Pfizer agreed to pay $450 million.[24][25] The other litigation is based on United States Pat. No. 6,048,850[26] owned by University of Rochester, which claimed a method to treat pain without causing gastro-intestinal distress by selectively inhibiting COX-2. When the patent issued, the university sued Searle (later Pfizer) in a case called, University of Rochester v. G.D. Searle & Co., 358 F.3d 916 (Fed. Cir. 2004). The court ruled in favor of Searle in 2004, holding in essence that the university had claimed a method requiring, yet provided no written description of, a compound that could inhibit COX-2 and therefore the patent was invalid.[27][28]

In the course of the search for a specific inhibitor of the negative effects of prostaglandins which spared the positive effects, it was discovered that prostaglandins could indeed be separated into two general classes which could loosely be regarded as "good prostaglandins" and "bad prostaglandins", according to the structure of a particular enzyme involved in their biosynthesis, cyclooxygenase.

Prostaglandins whose synthesis involves the cyclooxygenase-I enzyme, or COX-1, are responsible for maintenance and protection of the gastrointestinal tract, while prostaglandins whose synthesis involves the cyclooxygenase-II enzyme, or COX-2, are responsible for inflammation and pain.

The existing non-steroidal anti-inflammatory drugs (NSAIDs) differ in their relative specificities for COX-2 and COX-1; while aspirin and ibuprofen inhibit COX-2 and COX-1 enzymes, other NSAIDs appear to have partial COX-2 specificity, particularly meloxicam (Mobic). Aspirin is ≈170-fold more potent in inhibiting COX-1 than COX-2.[29] Studies of meloxicam 7.5 mg per day for 23 days find a level of gastric injury similar to that of a placebo, and for meloxicam 15 mg per day a level of injury lower than that of other NSAIDs; however, in clinical practice meloxicam can still cause some ulcer complications.

Many COX-2-specific inhibitors have been removed from the U.S. market. As of December 2011, only Celebrex (generic name is celecoxib) is still available for purchase.

Valdecoxib and rofecoxib are about 300 times more potent at inhibiting COX-2, than COX-1, suggesting the possibility of relief from pain and inflammation, without gastrointestinal irritation, and promising to be a boon for those who had experienced such adverse effects previously or had comorbidities that could lead to such complications. Celecoxib is approximately 30 times more potent at inhibiting COX-2 than COX-1, with etoricoxib being 106 times more potent. Also, Tribulus terrestris was shown to have strong inhibitory activity on COX-2.[30]

Between 1996 and 2009, anesthesiologist Scott Reuben conducted clinical research on the use of COX-2 inhibitors, often in combination with gabapentin or pregabalin, for the prevention and treatment of post-surgical pain, which was found in 2009 to have been faked. Reuben pled guilty, paid fines and served six months in jail, and lost his medical license.[31] A 2009 review of systematic review articles used in evidence-based medicine found that while some reviews were no longer valid when the Reuben studies were removed, the conclusions in the majority of them remained unchanged.[32] The review found that the key Reuben claims that needed to be re-examined were "the absence of detrimental effects of coxibs on bone healing after spine surgery, the beneficial long-term outcome after preemptive administration of coxibs including an allegedly decreased incidence of chronic pain after surgery, and the analgesic efficacy of ketorolac or clonidine when added to local anesthetics for intravenous regional anesthesia."[32]

Celebrex (and other brand names for celecoxib) was introduced in 1999 and rapidly became the most frequently prescribed new drug in the United States. By October 2000, its US sales exceeded 100 million prescriptions per year for $3 billion, and was still rising. Sales of Celebrex alone reached $3.1 billion in 2001. A Spanish study found that between January 2000 and June 2001, 7% of NSAID prescriptions and 29% of NSAID expenditures were for COX-2 inhibitors. Over the period of the study, COX-2 inhibitors rose from 10.03% of total NSAIDs prescribed by specialty physicians to 29.79%, and from 1.52% to 10.78% of NSAIDs prescribed by primary care physicians (98.23% of NSAIDs and 94.61% of COX-2 inhibitors were prescribed by primary care physicians). For specialty physicians, rofecoxib and celecoxib were third and fifth most frequently prescribed NSAIDs but first and second in cost, respectively; for primary-care physicians they were ninth and twelfth most frequently prescribed NSAIDs and first and fourth in cost.

The cause of the rapid widespread acceptance of Celebrex and Vioxx by physicians was the publication of two large trials, the Celecoxib Long-term Arthritis Safety Study [33] CLASS study in JAMA, and the Vioxx Gastrointestinal Outcomes Research[34] VIGOR study in the New England Journal of Medicine. Both publications concluded that COX-2 specific NSAIDs were associated with significantly fewer adverse gastrointestinal effects. In the CLASS trial comparing Celebrex 800 mg/day to ibuprofen 2400 mg/day and diclofenac 150 mg/day for osteoarthritis or rheumatoid arthritis for six months, Celebrex was significantly associated with fewer upper gastrointestinal complications (0.44% vs. 1.27%, P=0.04), with no significant difference in incidence of cardiovascular events in patients not taking aspirin for cardiovascular prophylaxis. In the VIGOR trial testing Vioxx 50 mg/day versus naproxen for rheumatoid arthritis, Vioxx reduced the risk of symptomatic ulcers and clinical upper gastrointestinal events (perforations, obstructions and bleeding) by 54%, to 1.4% from 3%, the risk of complicated upper gastrointestinal events (complicated perforations, obstructions and bleeding in the upper gastrointestinal tract) by 57%, and the risk of bleeding from anywhere in the gastrointestinal tract by 62%. An enormous marketing effort capitalized on these publications; Vioxx was the most heavily advertised prescription drug in 2000, and Celebrex the seventh, according to IMS Health.

Recent studies have shown that small tumors of the sympathetic nervous system (neuroblastoma) have abnormal levels of COX-2 expressed.[35] These studies report that overexpression of the COX-2 enzyme has an adverse effect on the tumor suppressor, p53. p53 is an apoptosis transcription factor normally found in the cytosol. When cellular DNA is damaged beyond repair, p53 is transported to the nucleus where it promotes p53 mediated apoptosis.[36] Two of the metabolites of COX-2, prostaglandin A2 (PGA2) and A1 (PGA1), when present in high quantities bind to p53 in the cytosol and inhibit its ability to cross into the nucleus. This essentially sequesters p53 in the cytosol and prevents apoptosis.[36] Coxibs such as Celebrex (celecoxib), by selectively inhibiting the overexpressed COX-2, allow p53 to work properly. Functional p53 allows DNA damaged neuroblastoma cells to commit suicide through apoptosis, halting tumor growth.

COX-2 up-regulation has also been linked to the phosphorylation and activation of the E3 ubiquitin ligase HDM2, a protein that mediates p53 ligation and tagged destruction, through ubiquitination.[36] The mechanism for this neuroblastoma HDM2 hyperactivity is unknown. Studies have shown that COX-2 inhibitors block the phosphorylation of HDM2 preventing its activation. In vitro, the use of COX-2 inhibitors lowers the level of active HDM2 found in neuroblastoma cells. The exact process of how COX-2 inhibitors block HDM2 phosphorylation is unknown, but this mediated reduction in active HDM2 concentration level restores the cellular p53 levels. After treatment with a COX-2 inhibitor, the restored p53 function allows DNA damaged neuroblastoma cells to commit suicide through apoptosis reducing the size of growth of the tumor.[36]